This invention pertains to a method to improve the selectivity of polymer membranes and their use for gas, vapor and liquid separations. More particularly, this invention pertains to a method to improve the selectivity of polybenzoxazole membranes prepared from aromatic polyimide membranes for gas, vapor, and liquid separations through the addition of a poly(styrene sulfonic acid) polymer in the preparation of polybenzoxazole membranes.
In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications have achieved commercial success, including carbon dioxide removal from natural gas and from biogas and enhanced oil recovery, and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams. For example, UOP's Separex™ cellulose acetate polymeric membrane is currently an international market leader for carbon dioxide removal from natural gas.
The membranes most commonly used in commercial gas separation applications are polymeric and nonporous. Separation is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The mechanism assumes that in a membrane having two opposing surfaces, each component is sorbed by the membrane at one surface, transported by a gas concentration gradient, and desorbed at the opposing surface. According to this solution-diffusion model, the membrane performance in separating a given pair of gases (e.g., CO2/CH4, O2/N2, H2/CH4) is determined by two parameters: the permeability coefficient (abbreviated hereinafter as PA) and the selectivity (αA/B). The PA is the product of the gas flux and the selective skin layer thickness of the membrane, divided by the pressure difference across the membrane. The αA/B is the ratio of the permeability coefficients of the two gases αA/B=PA/PB) where PA is the permeability of the more permeable gas and PB is the permeability of the less permeable gas. Gases can have high permeability coefficients because of a high solubility coefficient, a high diffusion coefficient, or because both coefficients are high. In general, the diffusion coefficient decreases while the solubility coefficient increases with an increase in the molecular size of the gas. In high performance polymer membranes, both high permeability and high selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given volume of gas, thereby decreasing capital cost of membrane units, and because higher selectivity results in a higher purity product gas.
Polymers provide a range of properties including low cost, good permeability, mechanical stability, and ease of processability that are important for gas separation. A polymer material with a high glass-transition temperature (Tg), high melting point, and high crystallinity is preferred. Glassy polymers (i.e., polymers at temperatures below their Tg) have stiffer polymer backbones and therefore let smaller molecules such as hydrogen and helium pass through more quickly, while larger molecules such as hydrocarbons pass through glassy polymers more slowly as compared to polymers with less stiff backbones. However, polymers which are more permeable are generally less selective than less permeable polymers. A general trade-off has always existed between permeability and selectivity (the so-called polymer upper bound limit). Over the past 30 years, substantial research effort has been directed to overcoming the limits imposed by this upper bound. Various polymers and techniques have been used, but without much success. In addition, traditional polymer membranes also have limitations in terms of thermal stability and contaminant resistance.
Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Currently, such CA membranes are used commercially for natural gas upgrading, including the removal of carbon dioxide. Although CA membranes have many advantages, they are limited in a number of properties including selectivity, permeability, and in chemical, thermal, and mechanical stability. It has been found that polymer membrane performance can deteriorate quickly. A primary cause of loss of membrane performance is liquid condensation on the membrane surface. Condensation can be prevented by providing a sufficient dew point margin for operation, based on the calculated dew point of the membrane product gas. UOP's MemGuard™ system, a regenerable adsorbent system that uses molecular sieves, was developed to remove water as well as heavy hydrocarbons from the natural gas stream, hence, to lower the dew point of the stream. The selective removal of heavy hydrocarbons by a pretreatment system can significantly improve the performance of the membranes. Although these pretreatment systems can effectively perform this function, the cost is quite significant. In some projects, the cost of the pretreatment system was as high as 10 to 40% of the total cost (pretreatment system and membrane system) depending on the feed composition. Reduction of the size of the pretreatment system or even total elimination of the pretreatment system would significantly reduce the membrane system cost for natural gas upgrading. Another factor is that, in recent years, more and more membrane systems have been installed in large offshore natural gas upgrading projects. The footprint is a big constraint for offshore projects. The footprint of the pretreatment system is very high at more than 10 to 50% of the footprint of the whole membrane system. Removal of the pretreatment system from the membrane system has great economic impact, especially to offshore projects.
High-performance polymers such as polyimides (PIs), poly(trimethylsilylpropyne) (PTMSP), and polytriazole have been developed to improve membrane selectivity, permeability, and thermal stability. These polymeric membrane materials have shown promising properties for separation of gas pairs such as CO2/CH4, O2/N2, H2/CH4, and propylene/propane (C3H6/C3H8). However, current polymeric membrane materials have reached a limit in their productivity-selectivity trade-off relationship. In addition, gas separation processes based on the use of glassy solution-diffusion membranes frequently suffer from plasticization of the polymer matrix by the sorbed penetrant molecules such as CO2 or C3H6. Plasticization of the polymer as demonstrated by membrane structure swelling and significant increases in the permeabilities of all components in the feed occurs above the plasticization pressure when the feed gas mixture contains condensable gases.
Aromatic polybenzoxazoles (PBOs), polybenzothiazoles (PBTs), and polybenzimidazoles (PBIs) are highly thermally stable ladderlike glassy polymers with flat, stiff, rigid-rod phenylene-heterocyclic ring units. The stiff, rigid ring units in such polymers pack efficiently, leaving very small penetrant-accessible free volume elements that are desirable to provide polymer membranes with both high permeability and high selectivity. These aromatic PBO, PBT, and PBI polymers, however, have poor solubility in common organic solvents, preventing them from being used for making polymer membranes by the most practical solvent casting method.
Thermal conversion of soluble aromatic polyimides containing pendent functional groups ortho to the heterocyclic imide nitrogen in the polymer backbone to aromatic polybenzoxazoles (PBDs) or polybenzothiazoles (PBTs) has been found to provide an alternative method for creating PBO or PBT polymer membranes that are difficult or impossible to obtain directly from PBO or PBT polymers by solvent casting method. (Tullos et al, MACROMOLECULES, 32, 3598 (1999)) A recent publication in the journal SCIENCE reported high permeability polybenzoxazole polymer membranes for gas separations (Ho Bum Park et al, SCIENCE 318, 254 (2007)). These polybenzoxazole membranes are prepared from high temperature thermal rearrangement of hydroxy-containing polyimide polymer membranes containing pendent hydroxyl groups ortho to the heterocyclic imide nitrogen. These polybenzoxazole polymer membranes exhibited extremely high CO2 permeability which is about 10-100 times better than conventional polymer membranes. However, the CO2/CH4 selectivities of these polybenzoxazole polymer membranes were similar to conventional polymer membranes. Improving the selectivities of these new polybenzoxazole polymer membranes is still needed to make new commercially viable membranes with higher separation efficiency than current commercial polymer membranes. The authors in this publication in the journal SCIENCE tried to increase the selectivity of these polybenzoxazole polymer membranes by adding small acid molecules (e.g., HCl and H3PO4). However, the stability of the small acid molecules in these polybenzoxazole polymer membranes has been found to be a critical issue for commercial use.
The present invention provides a method to improve the selectivity of polybenzoxazole membranes by the introduction of a polymer containing aromatic sulfonic acid groups such as poly(styrene sulfonic acid) polymer.